1. Field of the Invention
The present invention relates to a SAW (Surface Acoustic Wave) filter with an attenuation pole advantageously applicable to, e.g., a transmitter filter or a receiver filter included in a mobile communication terminal such as a cellular phone terminal.
2. Description of the Background Art
SAW filters with an attenuation pole for the above-mentioned application are disclosed in, e.g., Japanese patent laid-open publication Nos. 93382/1998 and 163808/1998, hereinafter referred to as Documents 1 and 2, respectively. Another SAW filter with an attenuation pole is proposed in Sato et al., “Small Loss, Band-Pass Filter Using SAW Resonators”, Papers A, the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J76-A, No. 2, pp. 245-252, 1993 (Document 3 hereinafter).
Small size, high performance parts are essential with cellular phones and other handy mobile communication terminals that are decreasing in size and weight. Specifically, there is an increasing demand for RF (Radio Frequency) parts using SAW devices.
Reference will be made to FIG. 2 for describing a ladder type SAW filter disclosed in Document 3. The ladder type SAW filter shown in FIG. 2 contributes a great deal to the miniaturization of the RF section of the terminals. SAW branching filters and other RF devices using the ladder type SAW filter have already been developed and partly put to practical use.
FIG. 3 plots an attenuation curve 201 and a return loss curve 202 particular to the SAW filter of FIG. 2, which is designed for an 800 MHz frequency band application. The horizontal and vertical axes indicate the attenuation in MHz and the frequency in dB, respectively. The curves 201 and 202 are derived from a serial arm resonator having 100 pairs with the transposition length of 100 micrometer and a parallel arm resonator having 70 pairs with the transposition length of 70 micrometer.
As shown in FIG. 4, the serial arm resonator has characteristics jx and rs in its imaginary number portion and real number portion, respectively. The parallel arm resonator has characteristics jb and rp in its imaginary number portion and real number portion, respectively.
The pass band shown in FIG. 3 ranges from around 863 MHz to around 911 MHz. As FIGS. 3 and 4 indicate, the attenuation range at the high frequency side (higher-frequency attenuation range hereinafter) of the above pass band has an attenuation pole at a point where the frequency of the serial arm resonator corresponds to infinity (about 42 dB), i.e., where the frequency is around 919 MHz. Also, the attenuation range at the low frequency side (lower-frequency attenuation range hereinafter) of the pass band has an attenuation pole at a point where the frequency of the parallel arm resonator corresponds to zero, i.e., where the frequency is around 855 MHz.
It is to be noted that FIG. 4 additionally shows the real number portions of the SAW resonators corresponding to a quality factor Q of 500.
As FIG. 3 indicates, the ladder type SAW filter shown in FIG. 2 has a single attenuation pole at each of the lower and higher frequency sides of the pass band. Therefore, the lower and higher frequency sides have substantially the same characteristic as each other, as well known in the art.
To meet the ever increasing demand for mobile communication terminals, the transmitter and receiver frequency bands are broadly allocated to both of a mobile communication system using a 800 MHz band and a mobile communication system using a 2 GHz band. The distance between the transmitter and receiver frequency bands is selected to be small in both of the above mobile communication systems.
For example, with the U.S. CDMA (Code Division Multiple Access) communication system used in the United States and allocating a frequency band of 824 MHz to 849 MHz to transmission and allocating a frequency band of 869 MHz to 894 MHz to receipt, the receiver frequency band is positioned in the higher-frequency attenuation range of the transmitter frequency band, so that the attenuation value does not have to be so great in the lower-frequency attenuation range. However, if the attenuation value in the higher-frequency attenuation range of the transmission band is small, then it is likely that a radio wave radiated from a mobile communication terminal turns round into the receiver band of the same terminal, lowering the reception quality.
To examine FIG. 3 from the above-mentioned point of view, let the attenuation curve 201 and return loss curve 202 be shifted to the left, i.e., the lower frequency side. Then, attenuation value of about −10 dB occurs in both of the higher-frequency and lower-frequency attenuation ranges. Such an attenuation value in the higher frequency range is not always enough.
By contrast, the SAW filter having the structure shown in FIG. 6, taught in, e.g., Document 1, implements a filter characteristic shown in FIG. 5. FIG. 6 shows a SAW filter with an attenuation pole LA including a ladder type SAW filter CP1, which has the configuration of a two-terminal pair circuit shown in FIG. 2, and another two-terminal pair circuit CP2 comprising a single inductor LX having inductance L. The two-terminal pair circuits CP1 and CP2 are serially connected to each other.
In FIG. 5, a downward arrow 1 indicates a point where the frequency and attenuation value are 818 MHz and −3.0609 dB, respectively. An upward arrow 2 indicates a point where the frequency and attenuation value are 843 MHz and −2.9886 dB, respectively. Further, an upward arrow 3 indicates a point where the frequency and attenuation value are 863 MHz and −43.794 dB, respectively. In addition, a further upward arrow 4 indicates a point where the frequency and attenuation values are 888 MHz and −38.099 dB, respectively.
It will be seen that a SAW filter having the characteristic shown in FIG. 5 implements a sufficient attenuation value in the higher-frequency attenuation range. Therefore, when such a SAW filter is applied to the transmitter band of the CDMA communication system used in the United States, it allows a minimum of radio wave to turn round into the receiver band and thereby enhances high quality transmission and reception.
However, even the filter characteristic shown in FIG. 5 cannot effect sufficient attenuation in the lower-frequency attenuation range. It follows that the filter characteristic available with the SAW filter taught in Document 1 is not sufficient when applied to the CDMA system in the United States as a receiver filter and is therefore likely to degrade the reception quality.
More specifically, if the SAW filter taught in Document 1 and lacking a good filter characteristic is applied to the CDMA system in the United State as a transmitter filter, then, the receiver filter fails to reduce the influence of the turn-round of a radio wave from the transmitter side of the same mobile communication terminal to a satisfactory degree. Further, the receiver filter cannot sufficiently reduce, e.g., the influence of an interference wave that may arrive at the mobile communication terminal from another radio communication apparatus.
As for systems other than the CDMA communication system in the United States, the frequency band allocated to transmission is sometimes higher than the frequency band allocated to reception. In such a case, the transmitter filter and receiver filter described above must be replaced with each other.
The SAW filters with an attenuation pole taught in Documents 1 and 2, whether they be transmitter or receiver filters, have the following problems left unsolved. In each of the SAW filters with an attenuation pole, a two-terminal pair circuit having a single inductance value L forms attenuation poles in a finite frequency range. When the attenuation poles vary, the attenuation value decreases in the lower-frequency attenuation range of the pass band and increases in the higher-frequency attenuation range of the same. It is therefore quite probable that the SAW filter will fail to satisfy a required standard in the lower-frequency attenuation range, although satisfying a required standard in the higher-frequency attenuation range.
The inductor LX, FIG. 6, may be provided with an inductance value L great enough to implement a sufficient attenuation value in the higher-frequency attenuation range. In practice, however, such a great inductance L is difficult to realize.
Moreover, if the distance between the attenuation range and the pass band, e.g., between the transmitter and receiver frequency bands of a single mobile communication terminal is as small as about 20 MHz, as stated earlier, then, the SAW filter needs a steep filter characteristic.